There has been considerable interest in the development of conjugates comprising various diagnostic or therapeutic agents attached to targeting proteins, such as antibodies. Recent efforts have included the conjugation of diagnostic, cytotoxic or anti-neoplastic agents to specific antibodies such as monoclonal antibodies to produce conjugates which can selectively target tumor cells while sparing normal tissues.
A large number of different classes of active agents have been considered, including diagnostic radioisotopes, beta and alpha emitting therapeutic radioisotopes, plant and bacterial toxins, and a variety of anti-neoplastic drugs including intercalating agents, antimetabolites, alkylating agents and antibiotics.
In spite of the advantages achieved by attaching active agents to targeting proteins, problems associated with localization of the conjugates on non-target organs remain. Targeting proteins such as antibodies are rarely totally specific for the desired target tissue, and conjugates comprising targeting proteins therefore frequently localize on non-target tissues through such mechanisms as cross-reactive binding, non-specific uptake, or excretory handling.
For example, one problem with the administration of radioimmunoconjugates used for diagnostic imaging, especially an immunoconjugate comprising a radionuclide imaging agent and an antibody fragment, is the tendency for radioactivity to localize nonspecifically in the kidney. This results in the kidney region frequently showing up on the image scans. Accordingly, a means for selectively removing the localized immunoconjugates from the kidney region without decreasing the radioactivity at the target site would be helpful for augmenting the clarity of diagnostic images. Further, an immunoconjugate comprising a therapeutic radionuclide moiety and an antibody fragment as the targeting protein generally will also localize in the kidney, potentially exerting undesirable renal toxicity and thereby lowering the therapeutic index of the immunoconjugate. Indeed, the kidney may be the limiting organ of toxicity, preventing administration of larger, more efficacious doses of radionuclide.
Other problems can develop when an immunoconjugate tends to localize in the liver. The liver occupies a large area in the abdomen, and localization of diagnostic immunoconjugates at the liver can mask other target sites where the immunoconjugate may have also localized, and obscure metastatic lesions. When a therapeutic immunoconjugate localizes in the liver, the liver tissue is exposed to the toxic effects of the therapeutic agent.
Efforts to effect clearance of radionuclide chelates that are not bound to target tissues in vivo have included the administration of immunoconjugates comprising radionuclide chelates attached to antibodies through certain linkers that are potentially cleavable under conditions present within the body. These potentially "metabolizable" linkers include certain linker molecules comprising thiourea groups, peptides, esters, or disulfides. See, for example, Quadri et al., (J. of Nuc. Med., Vol. 27, No. 6, p 959, Abstract No. 337, June 1986), Haseman et al. (J. Nucl. Med. 12:455-460 [1986]), and Meares et al. (Int. J. Cancer [Suppl.] U.S., Vol. 2, pp. 99-102[1988]).
Clearance of the radioisotope from the host's body (which is believed to be due to cleaving of the linker to release the chelate from the antibody) was analyzed. Clearance of radionuclides from the body generally was enhanced to various degrees, compared to immunoconjugates comprising non-cleavable linkers. When comparative studies were conducted, disulfide bonds generally were found to be more susceptible to cleavage in vivo than the other linkages described above. In fact, conventional disulfide linkers have been found to be too labile in many cases, with the chelates being released from the antibody so quickly that insufficient amounts of the immunoconjugates reached the target site. Much room for improvement remains with respect to the use of cleavable linkers to release chelates from immunoconjugates.
A need remains for methods of reducing localization of targeting protein conjugates comprising diagnostic or therapeutic agents on non-target tissues. Removing these agents from non-target organs will improve the clarity of diagnostic images and will reduce the exposure of non-target tissues to cytotoxic agents.